1. Field of the Invention
The invention relates generally to radio frequency combiner/divider devices.
2. Description of the Prior Art
There are many applications for reciprocal devices capable of combining radio frequency signals to achieve a higher power level or for dividing a given RF signal into equal parts and developing these divided signals at a corresponding plurality of branch ports. The latter may include signal division into a number of channels for separate signal processing.
Recently, solid state microwave amplifier modules capable of operation in the microwave regions have appeared in the art. These modules provide higher reliability but, compared to vacuum tube higher power amplifiers, are sharply limited in their power handling capability. Accordingly, power combining circuits are necessary in order that the inherent reliability of these solid state devices may be exploited while at the same time providing suitably high levels of transmittable power. It is also important that the failure of one or a few of the individual power generating modules not deteriorate the overall output power level available by significantly more than the contributions of the failed individual module or modules.
Prior art combiner/divider circuits exist for performing the basic power dividing and combining function aforementioned. For example, one relatively early version of such a device is described in a paper by E. J. Wilkinson entitled "N-Way Hybrid Power Divider" (IRE Transactions on Microwave Theory and Techniques, Volume MTT-8, pages 116-118 of January, 1960). That device has been identified in the art as the Wilkinson combiner/divider. Several disadvantages accrue to the Wilkinson combiner/divider, one of these being the fact that a star of balancing/isolation resistors is required and is difficult to realize in practice, particularly for a large number of output (branch) ports. The Wilkinson circuit causes the dissipation of considerable power in this resistor star (in the imbalanced mode), but the chip resistors usually employed for the purpose limit the power handling capability of the Wilkinson combiner/divider to less than 100 watts CW because adequate heat sinking of the resistors is simply not practical, especially at high frequencies.
Another form of power combiner/divider operable at higher power levels than the Wilkinson device is commonly referred to as the Gysel combiner/divider. That device was described in the IEEE-MTT-5 International Symposium Digest, page 116 (1975). The Gysel device has as its main advantage the capability of employing external isolation resistors which may be individually capable of higher power handling. Accordingly, the overall device is capable of higher power operation. Still further, the Gysel device provides for monitoring capability for imbalances at the output ports, but an important disadvantage of the Gysel combiner is the circuit construction restriction. Existing techniques use either overlapping two-layer stripline with inter-layer RF connections or a cylindrical cavity configuration. Either approach presents difficult mechanical problems especially at frequencies above 1.0 GHz.
One problem which occurs in so-called unattended or minimally attended radar transmitters using plural RF power generator modules and a combiner such as the Gysel device, is the effect of failure of an individual solid state module on the overall operation of the device. Not only is the power contribution of the failed module lost, but substantial loading of the outputs of the remaining operable modules results.
Neither the Wilkinson nor the Gysel combiner/divider configurations provide for continued operation without unacceptable overall losses when one or more of the transmitting modules becomes inoperative.
Another U.S. patent application which deals with the failed module problem in another way is U.S. Pat. No. 4,225,866, issued Sept. 30, 1980, and entitled "Automatic Failure-Resistant Radar Transmitter." That patent is likewise assigned to the present assignee. The system therein described allows for derated operation of the low power modules whose outputs are being combined for increased reliability and automatic increase of average power per module when a module failure occurs. The loading of the active modules inherent in that application is accepted as inevitable; however, an arrangement which would preclude those losses is much to be preferred.
The manner in which the present invention deals with the prior art disadvantages to produce a novel and advantageous combiner/divider structure will be understood as this description proceeds.